Abstract: The yellow supergiant content of nearby galaxies can provide a critical testof stellar evolution theory, bridging the gap between the hot, massive starsand the cool red supergiants. But, this region of the color-magnitude diagramis dominated by foreground contamination, requiring membership to somehow bedetermined. Fortunately, the large negative systemic velocity of M31, coupledto its high rotation rate, provides the means for separating the contaminatingforeground dwarfs from the bona fide yellow supergiants within M31. Using theMMT, we obtained spectra of about 2900 stars, selected using the color andmagnitude range to be yellow supergiants. Comparing the velocities to that ofM31-s rotation curve, we identified 54 certain, and 66 probable yellowsupergiants from among the sea of foreground dwarfs. We find excellentagreement between the location of yellow supergiants in the H-R diagram andthat predicted by the latest Geneva evolutionary tracks which include rotation.However, the relative number of yellow supergiants seen as a function of massvaries from that predicted by the models by a factor of more than 10, in thesense that more high mass yellow supergiants are predicted than are actuallyobserved. Comparing the total number of yellow supergiants with masses above20Mo with the estimated number of unevolved O stars indicates that the durationof the yellow supergiant phase is about 3000 years. This is consistent withwhat the 12Mo and 15Mo evolutionary tracks predict, but disagrees with the20,000-80,000 year time scales predicted by the models for higher masses.